There is an apparatus for estimating the state of a secondary battery, which is disclosed in patent document 1. The apparatus disclosed in patent document 1 estimates values of parameters representing a model of a secondary battery using an adaptive digital filter; the model is expressed as an equivalent circuit comprised of a single resistor and a single RC parallel circuit connected in series to the resistor.
Then, the apparatus estimates the state of charge of the secondary battery based on the estimated values of the parameters.